The Genoa case involves four attractive young women who allegedly struck up online friendships using social media, such as Facebook, Badoo and Chatroulette, and then enticed their victims into increasingly explicit sexual behavior to be recorded by webcam on Skype.

Police are investigating the possibility that the scam was organized by a male resident in another European country.

"The victims are of all ages and social classes," Belardini said in an interview at her office on the outskirts of Rome. "Many of them are ashamed and are reticent about reporting the blackmail attempt to the authorities."

The scammers seek to obtain the list of their victims' Facebook friends and then threaten to send the embarrassing videos to them unless they are paid. "They normally ask for €500 [US$693]," Belardini said.

The scammers sometimes tell their victims that they are under 18 years old and threaten to report them to the police for cultivating an inappropriate sexual relationship with a minor.

The police do not have centrally collated data on the number of blackmail complaints, since Italy's three police forces often

investigate crimes independently of one another, but this year there have already been nine complaints reported to the authorities in Bologna and a further 12 in the northern city of Bolzano.

Police advise victims not to pay and to report blackmail demands to the authorities, but they acknowledge that this relatively new form of cybercrime is hard to prosecute, and the punishments are far from draconian.

It can be difficult gathering the evidence of Internet and telephone traffic to prove a connection between the members of an extortion gang and particularly difficult to achieve results when someone is operating from outside Italy, Belardini said.

Suspects convicted of fraud risk a prison sentence of between six months and three years and a fine of up to about €1,000. If convicted of extortion the penalty could rise to a maximum of 10 years imprisonment and a €2,000 fine.

No one has yet been convicted of what the Italian press has dubbed "porno blackmail," and the ongoing investigations could take up to two years to complete, Belardini said.

In a somewhat similar case, however, a man from the central Italian town of Lanciano was sentenced in January to three years and four months imprisonment and a €3,300 financial penalty for attempting to extort money from a former lover by threatening to post images of their sexual trysts on the Internet.

Given the shame and embarrassment generated by this type of scam, investigators believe only a fraction of cases are coming to light.

Last August a 17-year-old English boy, Daniel Perry, committed suicide by jumping from a bridge after blackmailers threatened to make public his erotic Skype conversations with a person he believed to be a girl living in the U.S. state of Illinois.

The phenomenon has also been reported in New Zealand, where scammers are said to be demanding the equivalent of US $850 in return for a promise not to publish embarrassing Skype videos.

"I can guarantee you that there will be a lot of people falling for it," Detective Aaron Pascoe told the New Zealand Herald earlier this month. "They are asking for over (New Zealand) $1,000 and there will be people out there paying."

In a Web page urging people to "stay smart online," the Australian government has been warning against the danger of Skype video calls with strangers.

"During the video call the scammer may attempt to lead you into participating in intimate sexual activity or nudity, which can later be used to blackmail you," the site warns.

The site adds that Remote Access Trojans (RAT) "can remotely activate your webcam, at the same time disabling your camera indicator light. These images can also be used to blackmail you."

Bulk collection of webcam imagery by Britain's electronic surveillance agency GCHQ appears to have discovered, alongside putative terrorist threats, that "a surprising number of people use webcam conversations to show intimate parts of their body to the other person."

Britain's Guardian newspaper reported last month that the spy agency had targeted millions of Yahoo users' webcam images in a bulk collection program called Optic Nerve.

The information, provided by U.S. National Security Agency whistleblower Edward Snowden, included the unexpected revelation that between 3 percent and 11 percent of the Yahoo imagery contained "undesirable nudity".

GCHQ staff were advised not to open the material if it was likely to make them uncomfortable and were reminded that "dissemination of offensive material is a disciplinary offense."

The Italian postal police's Belardini warns that any images released onto the Internet should be considered as effectively in the public domain, so people need to be alert about what they post or allow to be recorded.

"This crime touches a very delicate aspect of people's personal sphere," she said. "People feel safe when they are in the privacy of their home, so their inhibitions can slip. You have to be very careful about what you share online."